The present invention relates to liquid stabilizer systems for vinyl halide resin compositions, to resinous compositions stabilized therewith, and to methods of preparation of such products. More particularly, it relates to liquid stabilizer systems including a tin-organo sulfur compound and an additive therefor.
It is well known that vinyl halide resins undergo undesirable changes when they are exposed to heat or to light, and that these changes lead to discoloration and to deterioration of the mechanical properties of compositions containing such resins. Since elevated temperatures are required for the processing and/or formulation of compositions containing these resins, and since the resins are exposed to heat, to light, or both, when they are subsequently used, it is necessary to incorporate in the vinyl halide compositions stabilizers that will inhibit or prevent discoloration, or loss of physical integrity, when they are exposed to such conditions.
Organo tin compounds that contain sulfur have long been recognized as highly effective heat stabilizers for vinyl halide resin compositions. Examples include organo tin ethanol mercaptides such as those disclosed in U.S. Pat. No. 4,059,562; and monomeric tetra-functional organotin mono- or di-alkyl di- or tri- (alkyl thioglycolate) or (mercapto alkyl carboxylate) compounds such as disclosed in U.S. Pat. No. 5,032,634.
A thorough historical review of organotin and organotin-sulfur compounds in stabilizing polyvinyl chloride compositions appears in the "Plastics Additives and Modifiers Handbook" (J. Edenbaum, Van Nostrand Reinhold, 1992, at pp. 309-326, Chapter 19: Dworkin, "Polyvinyl chloride processing stabilizers: tin and its derivatives".
Compounds of the formula (2) EQU (R.sup.1).sub.a Sn(--SCH.sub.2 COOR.sup.2).sub.4-a ( 2)
wherein the substituents R.sup.1 and R.sup.2 and the subscript a are as defined herein, are useful liquid stabilizers for vinyl halide resins, but have long been known to exhibit a tendency to form precipitates on standing. The precipitates generally have a higher tin content than the liquid product, but are considerably less effective as stabilizers. Thus, the formation of the precipitates represents an undesirable economic loss, in the stabilization efficiency which is lost, in the cost of the product which is lost to the precipiatate, as well as in the input of material and energy necessary to attempt to redissolve or otherwise handle the precipitates. There thus remains a need to formulate liquid stabilizers for vinyl halide compositions, including one or more components of the above mentioned formula (2), which do not exhibit a tendency to form precipitates and yet which otherwise can be formulated effectively into vinyl halide resin compositions.